1. Field of the Invention
The present invention relates to a combustion-powered nail gun that generates drive force by igniting a gas/air mixture to drive a nail into a work piece.
2. Description of Related Art
U.S. Pat. No. 5,197,646 discloses a conventional combustion-powered tool assembly. FIG. 1 schematically shows configuration of a conventional combustion-powered nail gun 1 similar to that disclosed in U.S. Pat. No. 5,197,646. The nail gun 1 includes a housing 14, a handle 11, a tail cover 17, a push lever 21, and a magazine 13.
The housing 14 accommodates therein a head cover 23, a combustion chamber wall 15, a cylinder 4, and a piston 10. The combustion chamber wall 15, the head cover 23, and the piston 10 together define a combustion chamber 5. The head cover 23 and the cylinder 4 are fixed with respect to the housing 14. The combustion chamber wall 15 is vertically movable within the housing 14 as guided by the housing 14 and the cylinder 4. Although not shown in the drawings, a connection rod linkingly connects the combustion chamber wall 15 with the push lever 21 so that the combustion chamber wall 15 and the push lever 21 move together in a ganged manner. Further, a spring (not shown) is provided for urging the push lever 21 downward. Therefore, the push lever 21 and the combustion chamber wall 15 are in their lower most position shown in FIG. 1 while no force operates against the urging force of the spring. At this time, because the head cover 23 and the cylinder 4 are fixed, an inlet 26 is opened between the head cover 23 and the combustion chamber wall 15 and an outlet 25 is opened between the cylinder 4 and the combustion chamber wall 15. Although not shown in the drawings, seals for forming a tight seal at the inlet 26 and the outlet 25 are provided at the lower end of the head cover 23 and the upper end of the cylinder 4. An intake vent 30 is provided in the upper end of the housing 14 and a discharge vent 32 is provided in the lower end of the housing 14.
The housing 14 further accommodates a motor 8, a spark plug 9, and a gas canister connection 7 in a space above the head cover 23. The gas canister connection 7 is connected to a gas canister (not shown) that hold combustible gas. An injection port 22 connects the gas canister connection 7 with the inside of the combustion chamber 5 and supplies combustible gas from the gas canister connection 7 into the combustion chamber 5. A fan 6 is disposed in the combustion chamber 5. The fan is attached to and rotated by the drive shaft of the motor 8. Electrodes of the spark plug 9 are exposed in the combustion chamber 5. Ribs 24 are provided on the inner surface of the combustion chamber wall 15 so as to protrude into the combustion chamber 5.
The piston 10 is supported by a slide seal member (not shown) so as to be vertically movable in the cylinder 4. A bumper 2 is provided below the piston 10 for absorbing excessive energy of the piston 10 after a nail driving operation. Also, an exhaust hole 3 is formed in the cylinder 4. A check valve (not shown) of well-known construction is provided on the exhaust hole 3 and the outer side of the exhaust hole 3.
The handle 11 is attached to a middle section of the housing 14. A trigger 12 is provided on the handle 11. Each time the trigger 12 is pulled (turned on), the spark plug 9 generates a spark.
The magazine 13 and the tail cover 17 are attached to the lower end of the housing 14. The magazine 13 is filled with nails (not shown). The magazine 13 feeds the nails one at a time to the tail cover 17. The tail cover 17 sets the nails fed from the magazine 13 in a position below the piston 10 and guides movement of the nails when the nails are driven downward by the piston 10.
FIG. 1 shows the nail gun 1 before a nail driving operation is performed. At this time, the push lever 21 is urged downward by the spring (not shown) to protrude below the lower end of the tail cover 17. The combustion chamber wall 15 is also in its lowermost position so that the inlet 26 is open between the combustion chamber wall 15 and the head cover 23 and the outlet 25 is open between the combustion chamber wall 15 and the cylinder 4. Also, the piston 10 is in its top dead position before a nail driving operation starts.
Next, a nail driving operation by the nail gun 1 will be described with reference to FIGS. 1 to 4. FIGS. 1 to 4 show changes in the nail gun 1 in chronological order when a nail driving operation is performed.
To prepare to drive a nail into a work piece 27, the user grips the handle 11 and presses the push lever 21 against the work piece 27 as shown in FIG. 2. As a result the push lever 21 rises upward against the urging force of the spring and the combustion chamber wall 15, being connected to the push lever 21, moves upward also into the position shown in FIG. 2. When the combustion chamber wall 15 moves upward in this manner, the inlet 26 and the outlet 25, which are above and below the combustion chamber wall 15 respectively, close up to seal close the combustion chamber 5 with the seals (not shown). In a linked operation, the gas canister connection 7 is pressed and so supplies combustible gas from the gas canister (not shown) to the injection port 22, which injects the combustible gas into the combustion chamber 5. Further, the motor 8 is turned on to rotate the fan 6. The injected combustible gas and air in the combustion chamber 5 are agitated and mixed together by rotation of the fan 6 in the sealed off combustion chamber 5 and influence of the ribs 24 that protrude into the combustion chamber 5.
Next, the user pulls the trigger 12 on the handle 11 to generate a spark at the spark plug 9. The spark ignites and explodes the air/gas mix in the combustion chamber 5. The gas expands as a result. The expanding gas drives the piston 10 downward as shown in FIG. 3 to drive the nail that is set in the tail cover 17 into the work piece 27.
Directly after combustion, the combusted gas that remains in the cylinder 4 and the combustion chamber 5 is extremely hot and in a high pressure state from having expanded. Because, as shown in FIG. 3, the piston 10 is in contact with the bumper 2 at a position below the exhaust hole 3, the combusted gas from the combustion chamber 5 flows through the exhaust hole 3 to outside of the cylinder 4 until the pressure in the cylinder 4 and the combustion chamber 5 reaches atmospheric pressure, whereupon the check valve in the exhaust hole 3 closes shut. During this time, the inner surface of the cylinder 4 and the inner surface of the combustion chamber wall 15 absorb the heat of the combusted gas so that the combusted gas rapidly cools and contracts. Therefore, after the check valve (not shown) closes, pressure in the thus sealed combustion chamber 5 above the piston 10 decreases to below atmospheric pressure. This is referred to as a thermal vacuum. This thermal vacuum pulls the piston 10 back to the upper dead position of before the nail driving operation.
After the nail is driven into the work piece 27, the user releases the trigger 12 and lifts the nail gun 1 upward away from the work piece 27. When the push lever 21 separates from the work piece 27, the spring (not shown) urges the push lever 21 and the combustion chamber wall 15 back into the positions shown in FIG. 4. Even after the trigger 12 is released and turned off, a control unit (not shown) continues rotation of the fan 6 for a fixed period of time to scavenge the combusted gas in the combustion chamber 5. That is, in the condition shown in FIG. 4, the inlet 26 and the outlet 25 are opened up above and below the combustion chamber wall 15 respectively. The combusted gas in the combustion chamber 5 is scavenged by rotation of the fan 6, which generates an air flow 16 that draws clean air in through the intake vent 30 and that exhausts combusted gas from the discharge vent 32. After the scavenging operation, the fan 6 is stopped. At this point, the nail gun 1 has returned to the initial condition shown in FIG. 1.
FIG. 5 shows the nail gun 1 after a nail driving operation. In this condition, the piston 10 is in its initial upper dead position and the fan 6 is generating the air flow 16 to scavenge the combusted air. During scavenging, a circulating flow 28 is generated that flows through a space S between the outer surface of the combustion chamber wall 15 and the inner surface of the housing 14. The circulating flow 28 returns a portion of the combusted gas back into the combustion chamber 5. As a result, a longer time is required to completely scavenge the combusted air. More time is required between successive nail driving operations, so that overall a series of nail drives takes longer. Efficiency of work using the nail gun 1 suffers.
FIG. 6 shows a modification of the conventional nail gun 1 wherein the intake port 30 is position adjacent to the inlet 26 and the discharge vent 32 is position adjacent to the outlet 25. This configuration reduces the resistance to the air flow 16 generated by the fan 6. However, a negative pressure develops in an area A near the inlet 26 due to the fan 6. Also, a positive pressure develops in an area B near the outlet 25 below the combustion chamber wall 15. This pressure difference generates the circulating flow 28. From this modification, it can be understood that the problem of the circulating flow 28 cannot be solved by merely changing the location of the intake and exhaust vents.
It is an objective of the present invention to eliminate or at lest greatly reduce this circulation flow in order to reduce the time required for scavenging and enable nail drive operations to be made rapidly in succession.
To achieve the above-described objective, a combustion-powered tool according to the present invention includes a housing, a push lever, a combustion chamber, a cylinder, a piston, an item setting unit, a gas injection unit, a drive start unit, and a blocking member.
The housing has an upper end, a lower end, an inner surface, and an outer surface.
The push lever is supported at the lower end of the housing.
The combustion chamber wall is disposed within the housing. The combustion chamber wall has an inner surface, an outer surface, an upper end, and a lower end. The inner surface of the combustion chamber wall substantially defines a combustion chamber. The outer surface of the combustion chamber wall is in confrontation with the inner surface of the housing and is separated from the inner surface of the housing by a space. The combustion chamber wall has an inlet in the upper end and an outlet in the lower end. The combustion chamber wall moves vertically within the housing in a ganged manner with the push lever between an open position wherein the inlet and the outlet are opened and a sealed position wherein the inlet and the outlet are closed.
The cylinder is disposed below and is in fluid communication with the combustion chamber. The piston is disposed in the cylinder and partially defines the combustion chamber with the combustion chamber wall. The piston is capable of vertical movement guided by the cylinder. The item setting unit is disposed at the lower end of the housing and sets the item in a position below the piston. The gas injection unit injects combustible gas into the combustion chamber.
The drive start unit ignites and explodes the combustible gas injected into the combustion chamber. The piston is driven downward in the cylinder by resultant expansion of gas in the combustion chamber and drives the item set in the item setting unit downward.
The blocking member blocks air from flowing between the upper end of the housing and the lower end of the combustion chamber wall through the space between the inner surface of the housing and outer surface of the combustion chamber.